1. Field of Technology
The invention relates to the field of microfabricated gryoscopes with wineglass structures.
2. Description of the Prior Art
Micro-wineglass gyroscope architectures along with fabrication processes relying on micro-glassblowing were disclosed previously in U.S. patent application Ser. No. 14/548,237 and Ser. No. 13/838,132, both incorporated herein by reference. These gyroscope architectures has potential for very high performance due to structural symmetry, low internal dissipation due to highly pure fused silica material and self-aligned stem structure, structural rigidity due to three dimensional geometry. For this reason, micro-wineglass gyroscope architectures are being pursued by many groups. One of the key performance elements of the gyroscope is the self-aligned stem structure which decouples the vibratory element from the substrate, essentially providing anchor loss mitigation (low internal dissipation) and providing immunity to package stresses. However, as the stem structure being the only attachment point of the resonator to the package, it is the weakest point of the gyroscope. This Achilles Heel of the gyroscope is typically a source of large stress concentration and structural deformation in the presence of environmental vibrations and shock, greatly diminishing the vibration immunity of the gyroscope. In addition, in order to achieve the highest performance possible, micro-wineglass gyroscopes need to be operated in vacuum to mitigate the effects of viscous air damping and change in environmental conditions, such as temperature, humidity and pressure changes. This often requires a hermetically sealed package encompassing the gyroscope to shield the gyroscope away from environmental factors and the outside environment. Even though wafer-level sealing techniques for conventional two dimensional gyroscopes do exist, a co-fabricated packaging process for sealing three dimensional micro-wineglass gyroscopes has not been demonstrated yet.
There have been two main prior approaches: (1) micro-wineglass gyroscopes fabricated through deposition of thin films onto pre-defined molds and (2) micro-wineglass gyroscopes fabricated through plastic deformation of bulk materials.
Consider first surface micro-machined micro-wineglasss gyroscopes. Surface-micromachined micro-wineglass gyroscopes are almost exclusively fabricated by depositing a thin film onto a pre-defined mold with a sacrificial layer to create the resonator element. Due to the nature of thin film deposition processes, they typically exhibit a small size (<1-2 mm diameter) and thin structures (<5 μm thickness).
Isotropic wet etching of silicon molds using HF—HNO3 and silicon nitride molds have been investigated at Cornell University, with the goal of depositing a thin film material (i.e., silicon nitride) into the mold at a later step to create hemispherical shell structures. Authors experimented with different HF and HNO3 ratios as well as different silicon orientations, <100> and <111> wafers. The mold isotropy was analyzed using optical profilometry. The level of anisotropy was measured using optical profilometry, due to the crystalline nature of silicon the hemispherical molds were deformed towards a square shape for <100> silicon and towards a hexagonal shape for <111> silicon. Lowest measured anisotropy of 1.4% was obtained for <111> silicon wafers using higher HF:HNO3 ratios. The process was later used to fabricate opto-mechanical light transducers.
Hemispherical shell structures were fabricated at Georgia Institute of Technology by isotropically etching silicon cavities, thermally growing SiO2 inside and later removing the silicon mold using XeF2 etching. As opposed to wet etching, the molds were created using a dry etching process (SF6 plasma etching). A radial deviation of 3.37 μm along the perimeter at a diameter of 1105 μm was reported. Hemispherical shell structures were subsequently coated with TiN using atomic layer deposition (ALD) for electrical conductivity. Electrostatic testing revealed a resonant mode with a Q-factor of about 6000 at 113 kHz. Similar structures were also fabricated out of polysilicon with integrated electrostatic transducers by using the SiO2 layer inside the mold cavity as a sacrificial layer and depositing polysilicon on top to create the device layer. A Q-factor of about 8,000 was observed at 421 kHz for these structures. Fabrication effect of thickness anisotropy on oxide micro-hemispherical shell resonators was analyzed using finite element analysis (FEA). Experimental results showed a frequency split (Δf) of about 94 Hz, the thickness anisotropy was associated to different oxide growth rates at different crystalline planes of the silicon wafer.
Poly-crystalline diamond hemispherical shell structures were fabricated at University of California, Davis by depositing poly-diamond thin films into hemispherical molds on a silicon wafer. Primary advantages of polycrystaline diamond films are potentially high Q-factor and potential for boron doping, creating inherently conductive shell structures, bypassing the need for an additional metal layer. Instead of wet/dry etching, the hemispherical molds were created by μ-EDM (electro-discharge machining), followed by HNA (HF, nitric acid, acetic acid) wet etching to smooth the mold surface. A piezo-electric shaker was used to excite the diamond hemispherical shell structures. Frequency sweeps using this method revealed a Q-factor of about 3,000 at 35 kHz. A frequency split (Δf) of about 1 kHz was observed between two degenerate n=2 wineglass modes (5% relative split). The frequency split was associated with the roughness at the rim of the shell structures. Later, frequency splits (Δf) as low as 5 Hz were reported. Rate gyroscope operation was demonstrated. More recently, a DRIE etched cylindrical mold was used to create poly-diamond cylindrical resonators demonstrating Q-factors in excess of 300,000 and frequency splits as low as 3 Hz.
Another poly-diamond hemispherical resonator gyroscope was reported at Charles Stark Draper Laboratory. In this research, wet etching of Corning 1715 glass was used to achieve highly isotropic cavities compatible with temperatures required for poly-diamond deposition, while retaining a closer coefficient of expansion match (CTE) to the poly-diamond structure. Using this technique, average cavity diameters of 1288 μm were etched, with perfect roundness within the resolution of the measurement (±0.5 μm). Q-factors as high as 20,000 were reported on n=2 wineglass modes.
Another SiO2 hemispherical shell fabrication process was reported by University of Utah, this process also relies on isotropically etched hemispherical molds on a silicon wafer. Thermally grown SiO2 was used as an etch stop layer along with a poly-silicon sacrificial layer underneath the oxide shells. Piezo-actuation and electrostatic drive using a probe tip were used for testing with laser Doppler vibrometry pick-off. Later electrostatic transduction and Q-factors above 10,000 at 22 kHz center frequency were reported.
In addition, thin film sputtered ULE (Ultra Low Expansion Glass) shells were reported using a process called ‘Poached-Egg Micro-molding’. As opposed to using hemispherical molds on a silicon wafers, the authors utilized precision ball lenses as a mold. The ball lenses were coated with a poly-silicon sacrificial layer followed by sputtering of ULE glass as the device layer. The coated ball lenses were placed onto silicon posts and the ULE above the equator line of the lens was etched using Ar plasma etching. Subsequently the ball lens was removed by etching the ULE above the equator of the ball lens and XeF2 of the poly-silicon device layer, leaving a sputtered ULE shell structure in the shape of the ball lens. Piezo shakers were used along with optical fiber pick-off for characterization. Q-factor of about 20,000 was observed at 17.3 kHz. Later, Silicon-on-Insulator (SOI) electrode structures were reported for electrostatic transduction.
All-dielectric (SiO2) cylindrical gyroscopes were reported by HRL Laboratories. The main difference from cylindrical resonators is the SiO2 resonator material. Transduction was achieved by using electric field gradients generated by interdigitated electrodes, eliminating the need for deposition of a conductive metal layer, which might potentially degrade the resonator performance. Q-factors as high as 12,000 were reported at 47.6 kHz center frequency using this technique.
Poly-crystalline diamond half-toroidal resonators were reported by Honeywell International. Resonators were fabricated by depositing poly-diamond onto micro-glassblown hemi-toroidal molds along with a polysilicon sacrificial layer. Frequency splits (Δf) as low as 2.4 Hz was reported on resonators with 2 mm diameter.
Extremely small (200 μm diameter) cenosphere-derived hemispherical shells were reported by University of Michigan. The shells are fabricated by ion-milling borosilicate glass cenospheres. For a sphere of 214 μm shell, quality factor of 130 was measured at 332.5 kHz.
Consider next bulk micro-machined micro-wineglasss gyroscopes. Next, we look at MEMS wineglass fabrication processes that rely on plastic deformation of bulk materials. A process based on ultrasonic machining (and EDM) is also considered.
Bulk metallic glass (BMG) spherical shells were fabricated at Yale University using blow molding. Platinum based (Pt57.5Cu14.7Ni5.3P22.5) bulk metallic glass with a processing temperature of 275° C. was used for the shell structures. Inert gases were used during most of the processing steps, due to low oxidation stability. Primary advantages of BMGs are low processing temperatures compared to most glasses, as well as inherent conductivity of the material, eliminating the need for metallization. Frequency splits as low as 5 Hz at 13.8 kHz and Q-factors as high as 7,800 at 9.4 kHz were demonstrated using this process.
Fused silica blow torch molding was used to create bird-bath (hemi-toroidal) and hemispherical shell structures at University of Michigan. To create the fused silica shells, thin layers of fused silica pieces were individually pressed onto graphite fixtures and deformed one at a time using the heat from a blow torch. Shells structures were later lapped from the back side to release the devices around their perimeter. Finally, the shells were sputter coated with thin layer of Ti/Au for conductivity. Ring-down testing under vacuum showed Q-factors as high as about 1.2 million at 8.7 kHz center frequency. Relative frequency split (Δfn=2/fn=2) ranged between 0.24% and 4.49% with a mean value at around 1-1.5% (100-150 Hz). Better alignment between the blow-torch and the mold as well as better temperature uniformity were proposed as a means to reduce the frequency split. At a later study, fused silica rods were embedded into the fused silica shell to create stem structures, showing Q-factors as high as 2.55 million at 22.6 kHz center frequency on uncoated resonators. Shells were later assembled into silicon-on-insulator (SOI) electrode structures to demonstrate rate gyroscope operation, showing angle random walk of 0:106°/√h and bias stability of 1°/h. In addition, a micro-machining process that utilizes ultrasonic machining (USM), electro-discharge machining (EDM), and lapping was proposed (3D-SOULE) to create micro-wineglass structures. EDM was mainly used to shape the stainless steel tooling, which was then used to USM fused silica spheres. Fused silica spherical-concave and mushroom type structures were created using this process. Laser Doppler Vibrometry was used to characterize the micro-wineglass structures, showing a Q-factor of 345 at 1.38 MHz in air.